How to middle align contents of a single column only?

Question

I am trying to use nicematrix to provide good illustration of presented material with good alignment of entries explained. I want the contents of

p1, p2, p3

to remain top aligned as shown. However, the contents of

p_{1} + p_{2} + p_{3} = 1

Must go exactly to the middle

My MWE

\documentclass{article}
\usepackage{calc}
\usepackage{nicematrix}
\newlength{\widthWidest}
\begin{document}
\setlength{\widthWidest}{\widthof{weight of constant impedance, constant current,}}
\begin{NiceTabularX}{\linewidth}{@{}r p[l]{\widthWidest} X[l, p]@{}}
$p_{1}, \medspace p_{2}, \medspace p_{3}$ & weight of constant impedance, constant current, and constant power components of the real portion of loads & $p_{1} + p_{2} + p_{3} = 1$

\CodeAfter
\SubMatrix{.}{1-1}{1-2}{\rbrace}


\end{NiceTabularX}
\end{document}

Answer 1

I see no reason for using the whole text width; to the contrary, I see reasons not to.

\documentclass{article}
\usepackage{lipsum}
\newlength{\auxlen}
\begin{document}
\lipsum[12][1-4]

[
\settowidth{\auxlen}{%
  $p_{1}$, $p_{2}$, $p_{3}$\hspace{6em}$p_{1} + p_{2} + p_{3} = 1$%
}
\left.
\begin{tabular}{@{} l p{\dimexpr\displaywidth-\auxlen} l @{}}
    $p_{1}$, $p_{2}$, $p_{3}$ &
    weight of constant impedance, constant current,
    and constant power components of the real portion of loads
\end{tabular}
\right}
p_{1} + p_{2} + p_{3} = 1
]
\lipsum[19][1-3]
\end{document}

A different proposal

\documentclass{article}
\usepackage{amsmath}
\usepackage{lipsum}
\begin{document}
\lipsum[12][1-4]

\begin{gather}
\left.
\begin{tabular}{@{} l l @{}}
$p_{1}$ & weight of constant impedance \
$p_{2}$ & constant current \
$p_{3}$ & constant power components
\end{tabular}
\right}
\text{of the real portion of loads}
\
p_{1} + p_{2} + p_{3} = 1
\end{gather}
\lipsum[19][1-3]
\end{document}

Answer 2

It's possible to produce what you want using the tabularray package by inserting the curly brace as a vertical line of the table.

\documentclass{article}
\usepackage{calc}
\usepackage{tabularray}
\newlength{\widthWidest}
\begin{document}
\setlength{\widthWidest}{\widthof{weight of constant impedance, constant current,}}
\begin{tblr}{
    width=\linewidth,   
    colspec={@{}r h{\widthWidest} X[l, m]@{}},
    column{1}={h, mode=math},
    column{2}={rightsep=0pt},
    column{3}={mode=math},
    vline{3} = {text=$\Bigg\}$}
}
    p_{1}, \medspace p_{2}, \medspace p_{3} & weight of constant impedance, constant current, and constant power components of the real portion of loads & p_{1} + p_{2} + p_{3} = 1
\end{tblr}

\end{document}

Answer 3

With the latest version of nicematrix (6.14 of 2023/02/18).

\documentclass{article}
\usepackage{calc}
\usepackage{nicematrix}
\newlength{\widthWidest}
\begin{document}
\setlength{\widthWidest}{\widthof{weight of constant impedance, constant current,}}
\begin{NiceTabularX}{\linewidth}{@{}r p[l]{\widthWidest} X[l, p]@{}}
$p_{1}, \medspace p_{2}, \medspace p_{3}$ & weight of constant impedance, constant current, and constant power components of the real portion of loads & \Block[v-center]{}{$p_{1} + p_{2} + p_{3} = 1$}
\CodeAfter
\SubMatrix{.}{1-1}{1-2}{\rbrace}

\end{NiceTabularX}
\end{document}

As usual with nicematrix, you need several compilations.